Endothelium-derived relaxing factor (EDRF) is a vascular relaxing factor secreted by the endothelium and is important in the control of vascular tone, blood pressure, inhibition of platelet aggregation, inhibition of platelet adhesion, inhibition of mitogenesis, inhibition of proliferation of cultured vascular smooth muscle, inhibition of leukocyte adherence and prevention of thrombosis. EDRF has been identified as nitric oxide (NO) or a closely related derivative thereof (Palmer et al, Nature, 327:524-526 (1987); Ignarro et al, Proc. Natl. Acad. Sci. USA, 84:9265-9269 (1987)).
Removal of the endothelium is a potent stimulus for neointimal proliferation, a common mechanism underlying the restenosis of atherosclerotic vessels after balloon angioplasty (Liu et al., Circulation, 79:1374-1387 (1989); Fems et al., Science, 253:1129-1132 (1991)). Balloon arterial injury results in endothelial denudation and subsequent regrowth of dysfunctional endothelium (Saville, Analyst, 83:670-672 (1958)) that may contribute to the local smooth muscle cell proliferation and extracellular matrix production that result in reocclusion of the arterial lumen. Nitric oxide dilates blood vessels (Vallance et al., Lancet, 2:997-1000 (1989)), inhibits platelet activation and adhesion (Radomski et al., Br. J Pharmacol, 92:181-187 (1987)), and nitric oxide limits the proliferation of vascular smooth muscle cells in vitro (Garg et al., J. Clin. Invest., 83:1774-1777 (1986)). Similarly, in animal models, suppression of platelet-derived mitogens decreases intimal proliferation (Fems et al., Science, 253:1129-1132 (1991)). The potential importance of endothelium-derived nitric oxide in the control of arterial remodeling after injury is further supported by recent preliminary reports in humans suggesting that systemic NO donors reduce angiographic restenosis six months after balloon angioplasty (The ACCORD Study Investigators, J. Am. Coil. Cardiol. 23:59A. (Abstr.) (1994)).
Another aspect of restenosis may simply be mechanical, e.g., caused by the elastic rebound of the arterial wall and/or by dissections in the vessel wall caused by the angioplasty procedure. These mechanical problems have been successfully addressed by the use of stents to tack-up dissections and prevent elastic rebound of the vessel thereby reducing the level of re-occlusion for many patients. The stent is typically inserted by catheter into a vascular lumen and expanded into contact with the diseased portion of the arterial wall, thereby providing internal support for the lumen. No material has, however, been developed that matches the blood-compatible surface of the endothelium. In fact, in the presence of blood and plasma proteins, artificial surfaces are an ideal setting for platelet deposition (Salzman et al, Phil. Trans. R. Soc. Lond., B294:389-398 (1981)). Exposure of blood to an artificial surface initiates reactions that lead to clotting or platelet adhesion and aggregation. Within seconds of blood contact, the artificial surface becomes coated with a layer of plasma proteins which serves as a new surface to which platelets readily adhere, become activated, and greatly accelerate thrombus formation (Forbes et al, Brit. Med. Bull., 34(2):201-207 (1978)).
Despite considerable efforts to develop nonthrombogenic materials, no synthetic material has been created that is free from this effect. In addition, the use of anticoagulant and platelet inhibition agents has been less than satisfactory in preventing adverse consequences resulting from the interaction between blood and artificial surfaces. Consequently, a significant need exists for the development of additional methods for preventing platelet deposition and thrombus formation on artificial surfaces.
There is a need in the art for effective methods of preventing and treating cardiovascular diseases and disorders, particularly, restenosis and atherosclerosis. The present invention is directed to these, as well as other, important ends.